Amorphous oxide semiconductor, semiconductor device, thin film transistor and display device

ABSTRACT

An amorphous oxide semiconductor contains at least one element selected from In, Ga, and Zn at an atomic ratio of InxGayZnz, wherein the density M of the amorphous oxide semiconductor is represented by the relational expression (1) below: 
       M≧0.94×(7.121 x +5.941 y +5.675 z )/( x+y+z )   (1)
         where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≠0.

RELATED APPLICATIONS

The present application is a continuation of application Ser. No.12/594,629, filed Oct. 5, 2009, which is a National Stage filing under35 U.S.C. §371 of International Application No. PCT/JP2008/057648, filedApr. 15, 2008. The present application claims benefit of parentapplication Ser. No. 12/594,629 (that is, PCT/JP2008/057648) under 35U.S.C. §120, and claims priority benefit under 35 U.S.C. §119 ofJapanese Patent Application 2007-115617, filed Apr. 25, 2007. The entirecontents of each of the mentioned prior applications are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to an amorphous oxide semiconductor, asemiconductor device, and a thin film transistor. In particular, thepresent invention relates to an amorphous oxide semiconductor containingat least one element selected from In, Ga, and Zn; and a semiconductordevice and a thin-film transistor employing the amorphous oxidesemiconductor.

BACKGROUND ART

Thin-film oxide semiconductors are being studied for use as a channellayer of a transistor. In particular, ZnO oxide-semiconductors are beingstudied actively for use as a channel of a thin-film transistor (TFT).

However, the thin ZnO film as a semiconductor film, when formed at roomtemperature, will become polycrystalline and will have severalnanometers to several tens of nanometers of roughness at asemiconductor-insulator interface, a semiconductor-semiconductorinterface, or a semiconductor-metal interface: the interface is highlyimportant in the semiconductor device.

Further, in the polycrystalline matter, crystal grain boundaries willinevitably be formed. The grain boundary will cause defects inconduction or will cause deterioration of the property owing toadsorption of gas molecules from the atmosphere to cause instability ofthe characteristics, disadvantageously (Journal of Applied Physics, Vol.94, p. 7748)

To overcome the aforementioned disadvantages resulting from the roughinterface or the grain boundaries, U.S. Patent Application PublicationNo. 2007/0194379 discloses a high-performance TFT which employs anamorphous oxide semiconductor as a thin-film of an oxide semiconductor.

The semiconductor formed in an amorphous state gives an excellentinterface having roughness of less than a nanometer without theroughness of the interface of ZnO, enabling a higher performance of thesemiconductor device. Thereby, a semiconductor film can be formedwithout grain boundaries, which prevents deterioration and instabilityof the properties caused by the grain boundaries.

Therefore, an amorphous oxide semiconductor film and a TFT containingthis semiconductor film were produced by pulse laser vapor deposition(PLD) under the same conditions as in the above U.S. Patent ApplicationPublication No. 2007/0194379. The obtained semiconductor film and theTFT had properties equivalent to the properties shown in that U.S.patent document, or reported in Nature, Vol. 432, p. 488.

Further, an amorphous oxide semiconductor film and TFT were produced bysputtering by use of a target of the composition InGaO₃(ZnO), namely anoxide of In₁Ga₁Zn₁. Thereby, an amorphous oxide semiconductor film and aTFT film were obtained which had as excellent properties as the oneproduced by the PLD process (Applied Physics Letters, Vol. 89, pages112123-1 to 112123-3).

After further investigation, the inventors of the present invention havefound the conditions for producing the amorphous oxide semiconductorhaving further improved properties of the semiconductor and TFT.

The present invention provides an amorphous oxide semiconductorcontaining at least one element selected from In, Ga, and Zn; and asemiconductor device and a thin-film transistor employing the amorphousoxide semiconductor.

DISCLOSURE OF THE INVENTION

The present invention is directed to an amorphous oxide semiconductorcontaining at least one element selected from In, Ga, and Zn at anatomic ratio of In_(x)Ga_(y)Zn_(z), wherein the density M of theamorphous oxide semiconductor is represented by the relationalexpression (1) below:

M≧0.94x(7.121x+5.941y+5.675z)/(x+y+z)   (1)

-   -   where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≠0.

In the amorphous oxide semiconductor, the relations x>0, y>0, and z>0can be accepted.

In the amorphous oxide semiconductor, the ratios x/(x+y+z), y/(x+y+z),and z/(x+y+z) can be respectively not less than 0.2.

The present invention is directed to a semiconductor device whichemploys the amorphous oxide semiconductor.

The present invention is directed to a thin film transistor whichemploys the amorphous oxide semiconductor as a channel layer.

The present invention enables production of an element having highperformance, high stability and high reliability for TFT employing anoxide semiconductor thin film as the channel layer.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the conduction properties of a bottom-gatetype TFT employing an amorphous oxide semiconductor of the presentinvention and of a conventional one.

FIG. 2 is a graph showing X-ray reflectivity of an amorphous oxidesemiconductor of the present invention and of a conventional one.

FIG. 3 illustrates a structure of the bottom-gate type TFT employed inExamples of the present invention.

FIG. 4 is a schematic illustration of a display device that mayincorporate a TFT according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The best mode of practicing the present invention is described withreference to the attached drawings.

Firstly the amorphous oxide semiconductor is described.

The amorphous oxide semiconductor of the present invention contains atleast one element selected from In, Ga, and Zn at an atomic ratiorepresented by In_(x)Ga_(y)Zn_(z).

In the composition, preferably x>0, y>0 and z>0, and more preferablyx/(x+y+z)≧0.2, y/(x+y+z)≧0.2, and z/(x+y+z)≧0.2. That is, the ratiosx/(x+y+z), y/(x+y+z), and z/(x+y+z) are preferably not less than 0.2respectively.

The amorphous oxide semiconductor may contain another oxide as anadditive, the additive including oxides of Mg, Ca, B, Al, Fe, Ru, Si,Ge, and Sn.

The amorphous oxide semiconductor of the present invention ischaracterized by the density thereof, which is not less than 94.0% ofthe theoretical density defined below.

In₂O₃ has a density of 7.121 g/cm³ according to PDF No. 00-006-0416,International Center for Diffraction Data.

Ga₂O₃ has a density of 5.941 g/cm³ according to PDF No. 00-041-1103,International Center for Diffraction Data.

ZnO has a density of 5.675 g/cm³ according to PDF No. 00-036-1451,International Center for Diffraction Data.

Accordingly, the theoretical density of the amorphous oxidesemiconductor containing In, Ga, and Zn at a composition ratio ofIn_(x)Ga_(y)Zn_(z) is calculated to be (7.121x+5.941y+5.675z)/(x+y+z)[g/cm³].

When the amorphous oxide semiconductor contains additionally an oxide ofan element other than In, Ga, and Zn, the theoretical density is definedas below. The added element is represented by M, and the compositionratio of In, Ga, Zn, and M is represented by In_(x)Ga_(y)Zn_(z)M_(w),and the density of the oxide having the lowest standard free energy offormation among the oxides of the element M is represented by D. Thenthe theoretical density is represented by(7.121x+5.941y+5.675z+Dw)/(x+y+z+w) [g/cm³].

The amorphous oxide semiconductor of the present invention has a densityof not less than 94% of the above theoretical density. This relation isexpressed by relational expression (1) below:

M≧0.94x(7.121x+5.941y+5.675z)/(x+y+z)   (1)

-   -   where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≠0.

Generally, a thin film of an amorphous substance has a density lowerthan the theoretical density owing to formation of void in the thin filmor another cause. For example, the amorphous silica film formed byvacuum vapor deposition as described in the Journal of Applied Physics,Vol. 78, pp. 962-968 has a density of 90% (2.0 g/cm³) of or lower thanthe density (2.2 g/cm³) of bulk silica. Reportedly, the film containsvoids capable of adsorbing moisture.

Such voids in the amorphous oxide semiconductor film can scatter thecarrier electrons or lengthen the conduction path to affect adverselythe conduction properties of the semiconductor such as the mobility.

To overcome such adverse effects, in the examples described below, theinventors of the present invention prepared an amorphous oxidesemiconductor film by a sputtering process, which enables readilyformation of a relatively high density of a thin film, by adjusting thefilm-forming temperature.

Thereby, a thin film of the amorphous oxide semiconductor was preparedwhich has a higher film density containing In, Ga, and Zn at acomposition ratio of In_(x)Ga_(y)Zn_(z). The thin film was found to besuperior to conventional ones in the conductivity characteristics,especially in the mobility.

The above-mentioned amorphous oxide semiconductor film having a densityof not less than 94.0% of the theoretical density is prepared preferablyby a vacuum vapor deposition including sputtering such as rf-sputteringand DC-sputtering, and pulse laser vapor deposition (PLD), especially bysputtering.

The temperature for formation of the above semiconductor is preferablynot lower than room temperature, more preferably in the range from 150°C. to crystallization-causing temperature.

In another preferred method of formation of the high-density amorphousoxide semiconductor film, firstly an amorphous oxide semiconductor filmhaving a density lower than 94.0% of the theoretic density is formed,and then the film is post-treated to increase the density to 94.0% orhigher of the theoretical density. The preferred post-treatment includesheat treatment, ion irradiation, plasma irradiation, and radicalirradiation.

The above amorphous oxide semiconductor is useful for a thin filmtransistor (TFT). The above amorphous oxide semiconductor is useful asthe channel layer of the TFT.

In the TFT, a Si oxide film or a metal oxide film is preferably employedas the gate-insulating film. The oxide for the gate-insulating film maycontain a small amount of nitrogen. A Si nitride is also preferred asthe gate-insulating film. The insulating film is formed preferably byvacuum vapor deposition including PLD (pulse vapor deposition) orsputtering.

The amorphous oxide semiconductor derived as described above hasproperties (e.g., mobility) more suitable for a semiconductor devicethan conventional amorphous oxide semiconductors. With the amorphousoxide semiconductor, a semiconductor device, especially a TFT, can beproduced having a higher performance than conventional semiconductordevice.

EXAMPLES Example 1 Amorphous Oxide Semiconductor

Firstly, an n-type Si substrate having thereon a thermally oxidized SiO₂film of 100 nm thick was prepared. On the SiO₂ film, an amorphous oxidesemiconductor film containing In, Ga, and Zn at a composition ratio ofIn_(x)Ga_(y)Zn_(z) was formed in a thickness of 40 nm at the substratetemperature of 200° C. by rf-sputtering with an InGaO₃(ZnO) target.

The composition of the formed film, namely x/(x+y+z), y/(x+y+z), andz/(x+y+z), was determined to be respectively 0.406, 0.376, and 0.218 byX-ray fluorescence analysis. From the composition analysis result, thetheoretical density was estimated at 6.36 g/cm³.

The obtained film was measured for X-ray reflectivity with an X-raydiffraction apparatus equipped with a Cu-Kα X-ray source and an X-rayminor.

FIG. 2 shows the measurement result by curve 21. Therefrom the filmdensity was estimated at 6.12 g/cm³, 96.2% of the above-estimatedtheoretical density.

Separately, another amorphous oxide semiconductor film of thecomposition ratio of In_(x)Ga_(y)Zn_(z) was formed by the same method ina thickness of 40 nm on the same substrate kept at room temperature(substrate temperature monitor reading: 22° C.).

The composition of the formed film, x/(x+y+z), y/(x+y+z), and z/(x+y+z),was determined to be respectively 0.397, 0.364, and 0.239 by X-rayfluorescence analysis. From the composition analysis result, thetheoretical density was estimated at 6.35 g/cm³.

The obtained film was measured for X-ray reflectivity. FIG. 2 shows themeasurement result by curve 22. Therefrom the film density was estimatedat 5.95 g/cm³, 93.8% of the above-estimated theoretical density.

As described later, TFTs were produced by use of the above amorphousoxide semiconductor film as the channel layer. The amorphous oxidesemiconductor films were tested for the field-effect mobility. The filmhaving a density of 96.2% of the theoretical density gave the mobilityof 12 cm²/Vs, whereas the film having a density of 93.8% of thetheoretical density gave the mobility of 5 cm²/Vs.

A film produced according to the method disclosed in the abovenon-patent literature 3 had a density of 93.7% of the theoreticaldensity, and a film produced according to the method disclosed in theabove non-patent literature 2 had a density of 83.7% of the theoreticaldensity. By comparison of the two films, the field-effect mobility ishigher in the film having the density of 93.7% of the theoreticaldensity.

From the above results, the density of the amorphous oxide semiconductoris correlative with the properties, especially the mobility: the higherthe density, the higher is the mobility.

In the present invention, it was found that the film having the densityof not lower than 94.0% of the theoretical density is superior in itsproperties to a conventional film of a lower density.

In the above Example, the amorphous oxide semiconductor film of a higherdensity was obtained by controlling the film formation conditions.

In another method, a film of a density of lower than 94.0% of thetheoretical density is formed firstly, and then the density is increasedby irradiation of ions, plasma, radicals, or the like. Thereby, anamorphous oxide semiconductor can be obtained which has a highperformance similar to that obtained by controlling the film formationconditions.

Example 2 Production of TFT Element

A bottom-gate type of TFT element illustrated in FIG. 3 was produced asa semiconductor device employing an amorphous oxide semiconductor film.

Firstly, an n-type Si substrate 101 having thereon a thermally oxidizedSiO₂ film 102 of 100 nm thick was prepared. On the SiO₂ film, amorphousoxide semiconductor film 103 containing In, Ga, and Zn at a compositionratio of In_(x)Ga_(y)Zn_(z) was formed in a thickness of 40 nm at thesubstrate temperature of 200° C. by rf-sputtering with an InGaO₃(ZnO)target. The obtained film had a density of 96.2% of the theoreticaldensity.

The obtained amorphous oxide semiconductor film containing In, Ga, andZn at a composition ratio of In_(x)Ga_(y)Zn_(z) was etched in anecessary size, and then source electrode 104 and drain electrode 105were formed by photolithography and a lift-off method. The material ofthe electrodes was a multilayer of Au (150 nm)/Ti (5 nm), and filmformation was conducted by electron-beam vapor deposition.

The n-Type Si 101, the substrate, was utilized as the gate electrode,and SiO₂ film 102 was utilized as the gate-insulating film of the TFTelement.

Among the obtained TFT elements, a TFT element having a channel lengthof 10 μm and a channel width of 30 μm was tested for conductionproperties (drain current/gate voltage characteristic, etc.). FIG. 1shows the characteristic as curve 11. The field-effect mobility was 12cm²/Vs.

Separately, in the same manner and constitution as above, an amorphousoxide semiconductor film of the composition ratio of In_(x)Ga_(y)Zn_(z)was formed on a substrate at room temperature (substrate temperaturemonitor reading: 22° C.) in a thickness of 40 nm (93.8% of thetheoretical density), and a TFT element was produced.

The TFT element having a channel length of 10 μm and a channel width of30 μm was tested for conduction properties. FIG. 1 shows thecharacteristic as curve 12. The field-effect mobility was 5 cm²/Vs.

As described above, a TFT element of higher performance was obtained byuse of an amorphous oxide semiconductor having a higher density as thechannel layer.

This high-density amorphous oxide semiconductor is effective not only inthe TFT but also in a device requiring a high mobility such as atransparent electrode material in an electronic device.

The amorphous oxide semiconductor containing In, Ga, and Zn in acomposition ratio of In_(x)Ga_(y)Zn_(z) of the present invention isuseful widely as a material for parts of an electronic device like adisplay (a schematic example 400 of which is illustrated in FIG. 4). Inparticular, the TFT employing this amorphous oxide semiconductor isuseful widely as a switching element for an LCD, and an organic ELdisplay, or the like, and a switching element of a switching element ofa matrix device such as a light-receiving element, and sensor element.Further the amorphous oxide semiconductor is useful for an IC card, anIC tag, or the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

1. A display device comprising a thin film transistor comprising: achannel layer; a source electrode electrically connected to the channellayer; a drain electrode electrically connected to the channel layer;and a gate electrode adjacent to the channel layer with a gateinsulating film interposed therebetween, the channel layer comprising anoxide semiconductor comprising at least one element selected from In,Ga, and Zn at an atomic ratio of InxGayZnz, wherein a mass density M ofthe oxide semiconductor is represented by a relational expression (1)shown below:M≧0.94x(7.121x+5.941y+5.675z)/(x+y+z)   (1) where 0≦x≦1, 0≦y≦1, 0≦z≦1,and x+y+z≠0.
 2. The display device according to claim 1, wherein x>0,y>0, and z>0.
 3. The display device according to claim 1, wherein ratiosx/(x+y+z), y/(x+y+z), and z/(x+y+z) are each not less than 0.2.
 4. Thedisplay device according to claim 1, wherein the oxide semiconductor isan amorphous oxide semiconductor.
 5. The display device according toclaim 1, wherein the display device is a liquid crystal display device.6. The display device according to claim 1, wherein the display deviceis an organic EL display device.